Acceleration switches are designed to switch on or off when a threshold acceleration value of acceleration is detected. An example of an application using acceleration switches is air bag systems. The acceleration switch in an airbag system will detect a sudden de-acceleration of the vehicle. When the threshold acceleration value is reached, the contacts on the switch close, sending a signal to the control module. Acceleration switches are also used in free fall detection systems for elevators, seat belt sensors, and machine monitoring of excess vibrations.
Acceleration switches were initially mechanical switches that included a spring loaded mass. Today, acceleration switches are more commonly manufactured using micromachining. Micromachining typically involves combining electronics and tiny mechanical components on a semiconductor chip. Initial designs used bulk micromachining, which consists of making micromechanical devices by etching into the silicon wafer. Bulk micromachining makes extensive use of wafer bonding, which is the process of permanently joining different silicon wafers together.
Further improvements in the manufacture of acceleration switches have been made using surface micromachining. Surface micromachining fabricates micromechanical devices on the surface of a silicon wafer. The features of the device are built up layer by layer through a combination of deposition, patterning, and etching stages. This technique is compatible with other semiconductor processing that may be performed on the same wafer for other purposes. Results from surface micromachining may be more uniform and more repeatable than those obtained from bulk micromachining.
Typical acceleration switches have contained at least one electrical contact. The contacts are designed to close when the threshold acceleration value is detected. The contacts are formed with a metal, such as gold. However, there may be problems with the use of metal contacts, such as microwelding, arcing, and oxidation. These problems may contribute to the failure of the switch.
It would be desirable to have an acceleration switch that does not employ a metal contact to signal when the threshold acceleration value is detected.